1. Field of the Invention
This invention relates to the field of video systems utilizing a liquid crystal display (LCD), and in particular, to video systems utilizing normally white liquid crystal on silicon imagers.
2. Description of Related Art
Liquid Crystal on Silicon (LCOS) can be thought of as one large liquid crystal placed over a silicon wafer. The silicon wafer is divided into an incremental array of tiny plates. A tiny incremental region of the liquid crystal is influenced by the electric field generated by each tiny plate and a common plate. Each such tiny plate and corresponding liquid crystal region are together referred to as a cell of the imager. Each cell corresponds to an individually controllable pixel. Each tiny plate is also a mirror for reflecting back a cell's light. A common plate electrode is disposed on the other side of the liquid crystal.
The drive voltages are supplied to plate electrodes on each side of the LCOS array. In the presently preferred LCOS system to which the inventive arrangements pertain, the common plate is always at a potential of 8 volts. Each of the other plates in the array of tiny plates is operated in two voltage ranges. For positive pictures, the voltage varies between 0 volts and 8 volts. For negative pictures the voltage varies between 8 volts and 16 volts.
The light supplied to the imager, and therefore supplied to each cell of the imager, is field polarized. Incoming light is incident upon the common electrode which is transparent. Each liquid crystal cell rotates the polarization of the input light responsive to the RMS value of the electric field applied to the cell by the plate electrodes. Generally speaking, the cells are not responsive to the polarity (positive or negative) of the applied electric field. Rather, the brightness of each pixel's cell is generally only a function of the rotation of the polarization of the light incident on the cell. Furthermore, polarization rotation for each cell is a non-linear function of the electric field. Polarization rotation for a given cell occurs as the light passes through the liquid crystal both before and after reflection from the cell plate. It is the rotation of the polarization that is capable of being controlled. Light leaving the imager is approximately the same intensity, but a different polarization. This may depend on the intensity that is ultimately desired. It should be noted that it is undesirable to have the imager absorbing light because it can get too hot. The imager will get hot due to some spurious amount of absorption.
If adjacent pixels produce different brightness, then there must be a different potential on the 2 cell plates corresponding to the adjacent pixels. When potentials on adjacent cell plates are unequal, there is an electric field between them which is known as a fringing field. The fringing field has some components, which are orthogonal to the desired field. These orthogonal components are not a problem in the space between adjacent mirrors. But, the orthogonal components of the electric field, which is over the mirror, will have the effect of distorting the polarization rotation. This distortion results in a substantial local increase in brightness. This is a particular problem when the pixel is supposed to be dark, but is usually an insignificant problem when the pixels are intended to be bright since the pixels are not very different in voltage so the fringing field is not that great. Also, for dark pixels, the additional brightness is much more noticeable. Contrast ratio is also very important in making a high quality display. It is very important to achieve sufficient black level. A proportionately larger drive voltage is needed to create a slightly darker image in a normally white display. Often, a large difference in voltage between adjacent pixels is needed. This results in a major fringing field that produces a visible artifact denoted sparkle. Due to the rotational effects of the fringing fields, this phenomenon is also referred to as a declination error in the imager. Sparkle artifacts can be red, blue and/or green, but green is usually the most prominent color.
Because of the particular manufacturing process used for many imagers, horizontally adjacent pixels suffer more from the fringing field problem. Thus, a need exists for overcoming the sparkle problem described above.